Multilayer thick film circuits have been used for many years to increase circuit functionality per unit of area. Moreover, recent advances in circuit technology have placed new demands on dielectric materials for this use. Heretofore, most of the dielectric materials used in multilayer circuits have been conventional thick film dielectric compositions. These are comprised of finely divided particles of dielectric solids and inorganic binders dispersed in an inert organic medium. Such thick film materials are usually applied by screen printing, though they may be applied by other means as well. Thick film materials of this type are very important and will continue to be so.
In constructing a multilayer circuit using thick film materials, it is necessary sequentially to print, dry and fire each functional layer before the next layer is applied. Thus, in a typical situation involving thick film multilayer circuits having, say, twenty layers, sixty separate processing steps are required as well as sixty inspections to assure the quality of each of the processed layers. Such a complex process is, of course, expensive both because of the great number of steps and because of the high yield losses which heretofore have been incident to such a complex procedure.
Another approach to this problem has been the use of dielectric tapes in which a large number of thin sheets of ceramic dielectric material, such as Al.sub.2 O.sub.3, are laid down interspersed with alternating printed layers of conductive materials. However, because of the very high temperature, on the order of 1600.degree. C., required to sinter Al.sub.2 O.sub.3, it is necessary to use very high melting conductive materials such as molybdenum and tungsten. Unfortunately, molybdenum and tungsten have only moderate conductivity properties which make them less satisfactory for very high speed, highly complex circuitry. Moreover, multilayer circuits made with these materials must be fired at 1600.degree. C. for quite long periods of time, which may approach 48 hours or more, to obtain adequate densification of the Al.sub.2 O.sub.3.
From the foregoing, it can be seen that there is a great need for a dielectric system which (1) can be fired at lower temperatures, thus permitting the use of conventional conductive materials such as gold, copper, silver and palladium, (2) can be densified by firing only a few hours, (3) can be fired in a nonoxidizing atmosphere with good burnout of organics and (4) give production yields having good adhesion to rigid substrates, layer integrity and thermal expansion characteristics closely resembling the substrate, which is usually fired alumina.